1. Field of the Invention
The invention relates to a method for operating a rolling mill, more particularly a strip mill, comprising, for example, a plurality of two-high or four-high roll stands in serial configuration.
2. Discussion of Related Art
The rolls associated with a rolling mill must be aligned parallel along the vertical and horizontal axis in order to produce a perfect product. It has been found, more particularly in strip rolling, that this requirement, which initially appears to be simple, can be satisfied only with substantial expenditure in terms of rolling mill precision in the light of rising demands made on the final product.
It is known that the difficulties become very great with very thin strips, since even a very slight offset in the shaft angle between the rolls results in changes of camber, which are large in relation to the strip thickness. Clearance alone between the chocks and the uprights of conventional four-high stands, due to bearing temperature rise, can amount up to 2 mm. As a result, the shaft angle of the rolls is offset in an extreme case by 4-5 mm and this can lead to a substantial change of pressure distribution in the roll gap. Initially, the operating personnel cannot readily recognise the resultant diagonal and transverse stresses in the tightly tensioned strip. It is only subsequently, in the absence of tension, that such stresses manifest themselves by bulges in the sheet metal.
Backing rolls and work rolls have been installed in offset configuration in the roll stand in accordance with the expected offset in the running direction of the rolled stock in order to counteract the offset of the roll shaft angles and therefore the stresses, which occur in the strip, without however obtaining a precise relative position between the roll axes in the subsequent rolling operation. The desired results could not be achieved even by the use of additional roll flexing devices.